Hello,
Just to clarify, what you want to do is take an existing GOG file, make some changes to it, and save a copy of that GOG file? Is that correct?
The quickest way to change an existing GOG file without overwriting the original would be to go outside of Drumagog, through Finder/Explorer to that particular folder, duplicate the GOG file you want to edit (with a different name), and make your edits to the duplicate GOG file.